PREFACE V the student's knowledge. By making this law the central idea of the entire subject and by gradually extending its interpretation the treatment is made uniform, coherent, and progressive. While appeal is made to the student's experience in introducing the principles of the conservation of dynamical energy and of the conservation of momentum they are shown to be direct consequences of the law of action and reaction. The equivalence and the alternative character of the conservation principles and of the law of action and reaction are emphasized by working out a number of problems by the application of both the law and the principles* The two types of motion, i.e., motion of translation and motion of rotation, are treated not only in the same general manner, but are developed along almost parallel lines. The simpler types of motion which are generally treated under Kinematics are given in the present work as problems in Dynamics. The author believes that the practice of divesting the physical character of the motion from the simpler types and reducing them to problems in integration is unfortunate. On account of their freedom from mathematical difficulties the simpler types of motion are particularly well adapted to illustrate the principles of dynamics. In order to differentiate between vectors and their magnitudes the former are printed in the Gothic type. In conclusion the author wishes to express his obligations to Mr. Leigh Page for reading the manuscript and to Dr, David D. Leib for reading the proofs and to both for many valuable suggestions. H. M. DADOUBIAN. YALTC UNIVERSITY, January) 1913.